Cervical cancer is the fourth most common cancer in women worldwide, with an annual incidence of 528,000 cases and mortality of 266,000 cases. Every year in the United States, there are 12,360 new cases of cervical cancer and 4,020 deaths. High-risk Human Papilloma virus, the most common type being HPV16, is the major cause of cervical cancer. Among the over one hundred different types of Human Papilloma virus, at least 15 are strongly associated with invasive squamous cell cancer of the cervix. HPV16 is the one most commonly found associated with this cancer.
Human Papilloma virus infection is also associated with the precursor lesion of cervical cancer, squamous intraepithelial lesion. While most low-grade squamous intraepithelial lesions prospectively regress spontaneously, some progress to high-grade squamous intraepithelial lesions. These high-grade lesions, in particular, cervical intraepithelial neoplasia-3 are associated with a high rate progression to invasive cervical cancer.
Two early gene products, E6 and E7, mediate transformation to a malignant phenotype by Human Papilloma virus. Both of these viral proteins have been shown to interact with the products of cellular human tumor suppressor genes. The E6 protein can bind and promote degradation of cell-encoded p53, while the E7 protein interacts with the retinoblastoma susceptibility gene product. Constitutive expression of HPV E6/E7 proteins is required for the maintenance of a malignant phenotype of cervical cancer.
Cell-mediated immunity plays an important role in controlling Human Papilloma virus infection and Human Papilloma virus-associated diseases. CD4 T cells are important in the development of anti-tumor responses. It is believed that the effectiveness of these CD4 T cells lies in their ability to deliver help for priming and maintaining CD8 cytotoxic T lymphocytes, which are thought to serve as the dominant effector cells in tumor elimination. Immunohistochemical analyses of squamous intraepithelial lesions and cervical cancer specimens have demonstrated the presence of activated cytotoxic T lymphocytes in lesions. The CD4 T cells activate cytotoxic T lymphocytes by producing T helper 1 cytokines and by providing activation signals for priming of tumor-specific cytotoxic T lymphocytes to professional antigen presenting cells. CD8-positive cytotoxic T lymphocytes recognize foreign peptides that are 8 to 11 amino acids in length and bound to and presented by Human Leukocyte Antigen class I molecules. These peptides are called T cell epitopes.
Memory T cells play an important role in maintaining long-term immunity to previously encountered pathogens or tumor antigens. They may proliferate, and rapidly acquire effector functions to kill virus-infected cells or tumor cells, and secrete cytokines that inhibit replication of the pathogen after re-stimulation with re-exposure to antigen. Antigen presenting cells, which may transfer peripheral antigenic signals to the lymphoid organs, play a crucial role in the induction of antigen-specific T cell immunity responses to Human Papilloma virus infection and Human Papilloma virus-associated tumors. Dendritic cells as professional antigen presenting cells express high level of major histocompatibility complex and co-stimulatory molecules. Insufficient or improper activation of dendritic cells, caused by lack of pro-inflammatory signal, leading to antigen presentation not in an appropriate co-stimulatory context is one reason for the failure of antitumor immunity.
Prophylactic HPV vaccines are available, and work by preventing HPV infection. But they are not effective in individuals who are already infected. An HPV therapeutic vaccine would benefit women who have pre-cancerous lesions but wish to have children since standard surgical treatments are associated with increased risk for pre-term delivery. It would also benefit women and men who live in developing regions of the world and do not have access to surgical modalities.